Non-puffing petroleum coke

ABSTRACT

Very fine particle size chromic oxide is dispersed in a high sulfur petroleum coker feedstock before or during delayed coking to produce a needle coke with low CTE and negligible puffing on heating to the temperature of graphitization.

This application is a continuation-in-part of Ser. No. 8,839, filed Feb.2, 1979, and now abandoned.

DESCRIPTION BACKGROUND OF THE INVENTION

Electrode grade graphite is manufactured from a commercial grade of cokehaving an acicular, anisotropic microstructure called needle coke, seeU.S. Pat. No. 2,775,549 to Shea, Dec. 25, 1956, Cl. 201-42, made bydelayed coking of certain petroleum residues under specific conditionsof heat and pressure. To produce graphite from such coke, it isnecessary to heat it to a temperature in the range of 2000°-3000° C.,which has the dual function of supplying energy for the conversion ofthe carbon in the coke to the graphitic crystalline form and ofvolatilizing impurities. When carbon bodies made from such cokes areheated at temperatures in the vicinity of 1000°-2000° C., varioussulfur-containing compounds decompose, attended by a rapid andirreversible expansion of the carbon body. This phenomenon is termed"puffing". During the production of graphite articles, particularly highperformance graphite electrodes, puffing is extremely undesirable as itmay destroy the structural integrity of the piece and render it marginalor useless for its intended purpose.

Puffing of a carbon article made from high sulfur cokes generally startsat about 1500° C., and may result in a volumetric expansion of as muchas 25%. It is not simply an elastic expansion but should becharacterized as an inelastic, irreversible expansion.

The generally accepted explanation of the puffing phenomenon is that inacicular needle cokes with a relatively large amount of sulfur, sulfuratoms are bonded to carbon atoms by covalent bonds, either in carbonring structures or linking rings. These bonds are less stable at hightemperatures than the carbon-to-carbon bonds. On heating, thecarbon-sulfur bonds rupture, the sulfur is freed, then reacts withhydrogen to form hydrogen sulfide. The simultaneous rupture of thesebonds and evolution of hydrogen sulfide and other sulfur containingmaterials causes the physical expansion called puffing.

Puffing has been avoided in the past by using coke made from petroleumresidues low in sulfur content. This approach is of only limited utilityat present since the principal petroleum crudes currently in use havehigh sulfur contents, and the cokes made from their residues such asdecant oil from catalytic cracking or thermal tar, will normally exhibitan undesirable degree of puffing.

Another approach to elimination or alleviation of the puffing problem inmanufacture of graphite articles has been by the use of additives. Theseadditives have usually been added during the mixing stage when varioussizes and grades of coke particles are mixed, before being wetted withpitch binder, formed into the desired shape, baked at an intermediatetemperature and graphitized at high temperatures. Additives haveincluded primarily metal salts and oxides, as disclosed in British Pat.No. 733,073, Greenhalgh, July 6, 1955, Cl. 90 b; French Pat. No.1,491,497, Gillot et al., Aug. 11, 1967, Cl. C 01 b; French Pat. No.2,035,273, Continental Oil, Dec. 18, 1970, Cl. C 10 b 57; U.S. Pat. No.3,642,962, Wallouch, Feb. 15, 1972, Cl. 201-17; U.S. Pat. No. 3,563,705,Grindstaff et al., Feb. 16, 1971, C. C 01 b 31/04, Cl. 423-375; U.S.Pat. No. 3,842,165, Grindstaff et al. Oct. 15, 1974, Cl. C 01 b 31/04,Cl. 264-29.1; and U.S. Pat. No. 3,338,993, Juel et al. Aug. 29, 1967,Cl. 106-56.

The patents above disclose the use of iron, sodium, chromium, nickel,cobalt, boron, aluminum, titanium, calcium, zirconium, manganese,magnesium, barium and strontium compounds as puffing inhibitors. Somecompounds of this group are in general usage and of these a choice isnaturally made based upon the effectiveness as a puffing inhibitor andupon other properties of the graphite article such as electricalresistivity, tensile strength, modulus of rupture, modulus ofelasticity, coefficient of thermal expansion, and cost.

Of the above, French Pat. No. 1,491,497 discloses the use of chromiumoxide at 0.2-5% in a mixture with coke and a binder as a catalyst,enabling graphitization to occur at temperatures in the range of1200°-2000° C.

French 2,035,273 discloses a low sulfur coke produced by the addition of0.3-5% of sodium carbonate to the coking stream mixture and subsequenthydrogenation of the coke at high temperature.

British 733,073 discloses the use of oxides of chromium, iron, copper,or nickel incorporated in the grinding stage of coke, mixed with pitch,shaped, baked at 1200° C., and graphitized at 2500°-2800° C.

U.S. Pat. No. 3,563,705 discloses the use of mixtures of iron or calciumcompounds with small amounts of titanium or zirconium compounds aspuffing inhibitors incorporated into the coke-binder mixture.

U.S. Pat. No. 3,338,993 discloses the use of calcium, magnesium,strontium, and barium fluorides as puffing inhibitors with raw orcalcined coke and binder, mixed, shaped, baked and graphitized.

U.S. Pat. No. 3,642,962 discloses the use of 1-3% calcium cyanamid orcalcium carbide as desulfurizing agents and puffing inhibitors, mixedwith raw coke prior to calcining.

U.S. Pat. No. 3,873,427, Long, issued Mar. 25, 1975, Cl. 201/17,discloses the addition of metallic chloride and ferruginous material fordesulfurization of coke.

U.S. Pat. No. 4,043,898, Kegler, issued Aug. 23, 1977, Cl. 208/50,discloses delayed coking of selected feedstocks to produce needle cokes.

At present, the most common methods of the above are those using ironoxides mixed dry in the coke-pitch binder blend as puffing inhibitors.These are effective puffing inhibitors but must be used with caution, astheir use tends to increase the coefficient of thermal expansion or CTE,of the finished product, to an undesirable level.

The coefficient of thermal expansion (CTE), which is conventionallyexpressed in in./in./°C. or cm./cm./°C., is also of vital importance inthe production of graphite for certain applications. Electrodes forelectric furnace melting of steel must have a low CTE to avoid excessivedifferential expansion at operating temperatures and the resultantspalling, which in turn causes excessive consumption of the electrode inoperation. Other applications requiring dimensional stability at hightemperatures are well-known although of somewhat less economicimportance.

In general, the addition of any foreign material to a graphitizingcarbonaceous mix will have, in addition to its desired effect, such aspuffing inhibition, the effect of increasing the CTE of the graphitebody.

A needle coke is distinguished by its physical structure whenmicroscopically examined, showing long needle-like acicular particles.Such cokes, to be suitable for manufacture of graphite electrodes to beused in ultra-high powered electric steel furnaces, should have agraphite CTE characteristic of less than 5+10⁻⁷ /°C. measured over therange of 0°-50° C. Needle cokes for lower powered electric steelfurnaces may have a graphite CTE characteristic of as much as 7×10⁻⁷/°C. over the 0°-50° C. range.

The cokes or blends of cokes must be thoroughly mixed with the puffinginhibitor to avoid the difficulties present in making uniformhomogeneous blends and to thoroughly coat the particles, which are oftenas much as 7 mm. in diameter. Both of these difficulties can lead tonon-uniform dispersion of the inhibitor and to puffing, even thoughthere is sufficient inhibitor present in the total mix to preventpuffing. This non-uniformity is particularly troublesome when operatingunder the newer type of graphitization processes, which raise thetemperature of the carbon bodies (i.e. electrodes) at a much higher ratethan the older processes. The combination of high sulfur with high rateof temperature rise exacerbates the problem and requires undesirablyslow heating rates to overcome puffing.

It should be emphasized that overcoming the puffing problem becomesincreasingly more difficult in the larger graphite electrode sizes(above 20 in. ((51 cm.)) diam) because in such sizes, larger particlesof coke are used. Since the puffing inhibitor only coats the surface ofthe particles, the coke surface area to inhibitor weight ratiodecreases, for a given weight addition ratio, giving a higherconcentration of inhibitor on the coke particle surfaces for the largerparticle blends. Thus a large amount of the inhibitor is at relativelygreater distance from the centers of the coke particles in the largercoke particle mixes as opposed to the smaller particle mixes used insmaller electrodes. Migration of the inhibitor into the centers of thelarge particles becomes progressively more difficult and less effectiveas the coke particles increase in size.

The puffing problem is further increased with the rate of graphitizationof the carbon bodies. Optimum distribution of the inhibitor throughoutthe structure of the carbon body to be graphitized is essential as thedegree of puffing for any coke particle blend is highly rate sensitive,being directly related to the rate of temperature increase during thegraphitization cycle. Thus, the figures in certain of the examples givenwill show a much higher dynamic puffing at a 14° C./min. temperaturerise than for a 5° C./min. rise.

The amount of puffing for any given coke-inhibitor blend could beexpressed as a proportionality in the general form:

    ______________________________________                                         ##STR1##         where S = sulfur content of coke                                             P = mean particle size                                                        ΔT = rate of temperature                               where DP = dynamic puffing                                                                     increase                                                                      1 = amount of inhibitor                                                       K = proportionality factor                                   ______________________________________                                    

Thus it may be seen that increases in sulfur content, particle size, andtemperature rise will increase puffing, while an increase in theinhibitor level will decrease puffing.

SUMMARY OF THE INVENTION

A petroleum coker feedstock which would normally produce a puffing cokedue to its high sulfur content is rendered non-puffing by the additionof an effective amount of a chromium compound, preferably chromic oxide,to the feedstock as a fine particle size powder.

The chromic oxide may be pre-dispersed in a high concentration in asmall quantity of the feedstock (fresh feed or coker furnace feed) orcompatible material miscible with the feedstock or dispersed in thetotal coker stream and added either batchwise to a batch type coker,continuously to the main stream in a delayed coker, or near the top orside of a delayed coker, as in the case of anti-foam additives, whilethe coker stream is admitted into the coker at or near the bottom of theunit.

The use of a fine particle size chromic oxide of 100% less than 5 micronand 70% less than 2 micron diameter, predispersed in a portion of thefeedstock, insures that the final product will be a homogeneous cokewith chromic oxide uniformly distributed throughout.

A current of inert gas or steam bubbled slowly through the hydrocarbonsin a batch type coker during the run aids in keeping the chromic oxidein suspension without significantly increasing the CTE of the finishedproduct. For a description of delayed coking, see R. J. Diwoky,Continuous Coking of Residuum by the Delayed Coking Process, Refiner andNatural Gasoline Manufacturer, Vol. 17, No. 11, Nov. 1938. The Cr₂ O₃dispersion may be injected through the anti-foam injector ports or aspecial fitting.

Many chromium compounds may be used, and the present disclosure is notmeant to limit the scope of the invention. Cr₂ O₃, for example is thefinal product of calcination of numerous chromium compounds, e.g.,hydrated Cr (III) oxide, Cr (III) nitrate, [Cr(NH₃)₆ ] (NO₃)₃, and othercomplex compounds and salts such as (NH₄)₂ Cr₂ O₇. Cr₂ O₃ ismanufactured commercially by ignition of compounds such as metalchromates and bichromates with reducing agents and may be produced insitu by reduction with the coke.

The reactive species may be elemental chromium, produced by reduction ofthe Cr₂ O₃ by the coke during the graphitization process, or bydissociation occurring at or below its melting point (2275° C.),similarly to the production of ferrochromium alloys by reduction ofchromite ore with coke in a submerged arc furnace.

The mode of operation of Cr₂ O₃ in inhibiting puffing has the mostlikely hypothesis of the reaction with a sulfur compound to form Cr₂ S₃in a scavenging reaction, the sulfide later decomposing at a highertemperature in a slower reaction.

In general the use of any of the additives listed above, when added to acoke particle-pitch binder mix, will lower the extent of puffing, but atthe same time significantly increase the CTE, of the graphite bodiesmade from such cokes. We have found that the use of chromium oxidedispersed in the coker feedstock when coking in a delayed coker gives anunexpected advantage in that it controls puffing of the coke whileincreasing the CTE only to a smaller degree (or in some instances not atall), when compared to the CTE of a graphite body in which the puffinghas been eliminated by adding the same additive to the electrode mix bythe conventional dry-mix practice.

CTE of the graphitized coke was determined by preparing small 5/8"×5"(1.6×12.7 cm.) electrodes by the procedure disclosed in U.S. Pat. No.2,775,549, (except for calcination of the coke to 1250° C.), andmeasuring their elongation over the temperature range of 0° to 50° C.

DESCRIPTION OF THE DRAWING

The drawing is a schematic illustration of apparatus used in carryingout the process.

In FIG. 1 a decant oil, the fractionater tower bottoms from acatalytically cracked gas oil fraction, also termed slurry oil, or otherequivalent hydrocarbon residue, is conveyed from the fractionater 33through line 10 and meter 14 to diversion valve 17, where a portion ofthe feedstock is diverted through valve 13, and meter 15 to disperser18. Simultaneously a portion of chromic oxide 12 is weighed in scale 16and conveyed to disperser 18 where it is dispersed in the feedstock to aspecific concentration by weight. Alternately a compatible liquid andadditives from supply 19 are metered through valve 11 to valve 13 andmeter 15 to disperser 18. The chromic oxide is dispersed and dischargedthrough line 22 and meter 23 to valve 34, line 36 and injection ports 38and 38A or alternately to mixer 24 where it is mixed with the mainportion of the feedstock coming through line 20 and meter 25, to theexact proportion desired. The chromic oxide concentrate mixed with thefeedstock is then conventionally delay coked. The overheads are takenoff through line 32 and sent to the fractionator 33.

In the above flowsheet, 18 is the disperser which may be any of severaltypes of equipment well known in the art, preferably a high shear orcolloid mill. Alternately, a sand or ball mill could be used.

In practice, a dispersion of approximately 3-50% by wt. of chromic oxidein the feedstock is used as a concentrate.

The chromic oxide dispersion and feedstock are metered in the correctproportions to give a concentration of approximately 0.05-0.5 wt. % Cr₂O₃ in the feedstock.

At the operating temperatures the viscosity of the feedstock isextremely low and some means is necessary to minimize settling and aconcentration of the chromic oxide in the lower portion of the cokerduring batchwise coker operation. We have found that by the introductionof a small flow of inert gas bubbled up from the bottom of the cokerdrum, the chromic oxide is maintained in a uniform suspension withoutsignificantly raising the CTE of the finished product or lowering theacicular crystal content of the coke. It is preferable to add the Cr₂ O₃at or near the top of a commercial delayed coker while admittingpreheated coker feed through or near the bottom of the coker.

The following are examples of specific methods of practicing theinvention:

EXAMPLE 1

The micronized puffing inhibitor, chromic oxide, was mixed with samplesof a fresh feed decant oil coker feedstock, at 0.1 wt. % level in a highspeed blender for about 5 minutes. The mixtures were coked underidentical conditions in 4 liter resin flasks.

In an insulated glass resin flask, an inert gas at the rate of 0.16SCFH/kg (4.5 l./hr./kg.) was bubbled up from the bottom of the cokingpot to keep the Cr₂ O₃ uniformly dispersed in feedstock. The followingtime-temperature cycle was used:

    ______________________________________                                        Temperature   Elapsed Time   Rate ΔC°/hr                         ______________________________________                                        Room to 350° C.                                                                      3 hours        110                                              350-450° C.                                                                          4.5-5 hours    20                                               450° C.                                                                              16 hours       --                                               450-530° C.                                                                          4.0-4.5 hours  20                                               530° C.                                                                              1 hour         --                                               530° C.-RT                                                                           Cool-down, power off                                            ______________________________________                                    

Dynamic puffing (DP) of the cokes was then determined by the methodbelow and compared with uninhibited samples, and with samples inhibitedin the normal manner with dry-mixed iron oxide. The coke samples had50%<200 mesh (78 mesh/cm.) particles and 100%<65 mesh (26 mesh/cm.)particles.

DP was measured by taking representative samples by the method of ASTMD346-35, crushing, mixing 100 g coke and 25 g pitch, and molding plugsat 12,500 psi (879 kg./cm.²). The plugs were measured by micrometer andplaced in a dilatometer. The temperature was raised to 1200° C. over aperiod of 50±10 min. The test was run at a temperature increase of 5° or12°-16° C./min. over the 1200°-2900° C. range, with measurements takenevery five minutes. The reported DP (dynamic puffing) is the maximumpercentage of elongation (or shrinkage) measured. All of the DP's belowwere at 14° C./min. rise except as noted.

    ______________________________________                                                  Coke Properties                                                     Inhibitor                                                                              Coke               CTE × 10.sup.-7 /°C.**                                                      DP, % Δ L***                     Added to Yld.   Ash    S    pph Fe.sub.2 O.sub.3 *                                                                   pph Fe.sub.2 O.sub.3 *                 Feedstock                                                                              %      %      %    0      1/2  0     1/2                             ______________________________________                                        0        23.3   0.01   1.17 3.0    --    6.8  --                              0        23.3   0.01   1.17 --     3.8   --   1.7                             0.1% Cr.sub.2 O.sub.3                                                                  22.6   0.46   1.07 3.8    --    -1.5 --                              ______________________________________                                         *(Iron oxide drymixed into the coke)                                          **(Coefficient of Thermal Expansion over the range of 0°-50.degree     C. × 10.sup.-7 /°C., 5/8" × 5" ((1.6 × 12.7 cm.)     sample, 10 min. time)                                                         ***(Dynamic Puffing over the range of 1200°-2900° C.)      

It is clear that micronized chromic oxide was found to be highlyeffective. The addition of micronized chromic oxide to the feedstockinhibited puffing of the resulting coke. While exhibiting lower DP, theresulting coke had a CTE (˜0.5% ash which is essentially chromic oxide)equal to that of the coke inhibited with an equal amount of iron oxidein a dry blend.

EXAMPLE 2

Identical to Example 1 except that 0.05 wt. % micronized chromic oxidewas added to the feedstock prior to coking with the results shown below:

    ______________________________________                                                  Coke Properties                                                     Inhibitor                                                                              Coke               CTE × 10.sup.-7 /°C.                                                        DP, % Δ L                        Added to Yld.   Ash    S    pph Fe.sub.2 O.sub.3 *                                                                   pph Fe.sub.2 O.sub.3 *                 Feedstock                                                                              %      %      %    0      1/4   0    1/2                             ______________________________________                                        0        23.3   0.01   1.17 3.0    3.6   6.8  1.7                             0.05% Cr.sub.2 O.sub.3                                                                 21.8   0.25   1.17 3.0    --    1.5  --                              ______________________________________                                         * dry blend                                                              

The above data indicate that the addition of micronized chromic oxide tothe coker feedstock resulted in a coke (0.25% ash) with a lower CTE anda lower puffing than that of the coke conventionally inhibited withcorresponding amounts of iron oxide, respectively.

EXAMPLE 3

Micronized chromic oxide (0.1 wt. %) was added to another sample offurnace feed decant oil coker feedstock. The mixture was coked in thesame manner described in Example 1:

    ______________________________________                                                 Coke Properties                                                      Inhibitor                                                                             Coke                CTE × 10.sup.-7 /°C.                                                        DP, % Δ L                        Added to                                                                              Yld.   Ash     S    pph Fe.sub.2 O.sub.3 *                                                                   pph Fe.sub.2 O.sub.3 *                 Feedstock                                                                             %      %       %    0      1     0    1                               ______________________________________                                        0       26.4   <0.01   0.97 3.3    3.5   +2.3 -1.8                            0.1%    25.6    0.35   0.98 2.6    --    -1.3 --                              Cr.sub.2 O.sub.3                                                              ______________________________________                                         *dry blend                                                               

The above data showed that micronized chromic oxide (0.1 wt. %) added tothe feedstock inhibited puffing of the resulting coke without adverselyaffecting the CTE of the coke. In fact, the CTE of the resulting coke(0.35% ash) was less than that of the coke with no inhibitor orconventionally inhibited with 1 pph iron oxide.

EXAMPLE 4

Micronized chromic oxide (0.075 wt. %) was dispersed in a sample ofslurry oil coker feedstock. The mixture was then coked using theprocedure of Example 1. The properties of the coke were determined forcomparison with that of the control coke from this oil with and withoutthe addition of micronized chromic oxide.

    ______________________________________                                                    Coke Properties                                                   Inhibitor                             DP, % Δ L                         Added to                                                                              Coke Yld. Ash     S    CTE    pph Cr.sub.2 O.sub.3 *                  Feedstock                                                                             %         %       %    10.sup.-7 /°C.                                                                0    1                                  ______________________________________                                        0       16.3      0.06    1.29 2.5    8.5  --                                 0       16.3      0.06    1.29 --     --   0                                  0.075%  16.7      0.51    1.36 2.2    0.7  --                                 ______________________________________                                         *Dry blend                                                               

It is seen from the above comparison that micronized chromic oxide addedto the feedstock resulted in a coke (0.51% ash) with a substantialreduction in puffing without adversely affecting the CTE of the coke. Infact, the CTE of the coke made in the presence of chromic oxide wasslightly lower than that from the original untreated feedstock.

EXAMPLE 5

Micronized chromic oxide was added conventionally by dry mixing to acoke sample made from the decant oil of Example 1, to determine itsrelative effectiveness in a dry blend vs. addition to the cokerfeedstock, to an equal Cr₂ O₃ concentration on the coke basis withrespect to puffing inhibition. Results were as follows:

    ______________________________________                                                Coke Properties                                                       Inhibitor                           DP, % Δ L                           Added to                   CTE      pph Cr.sub.2 O.sub.3                      Feedstock Ash %     S %    10.sup.-7 /°C.                                                                  0    1/2*                                 ______________________________________                                        0         0.01      1.17   3.0      --   0.1                                  0.1% Cr.sub.2 O.sub.3                                                                   0.46      1.07   3.8      -1.5 --                                   ______________________________________                                         *dry blend                                                               

The above data clearly indicate that micronized chromic oxide is moreeffective in the coker feedstock than when used conventionally as apuffing inhibitor.

Examples 1-5 above were made in 4 liter resin flasks and were agitatedby nitrogen bubbling for one minute when the temperature reached 420° C.No settling of chromic oxide was observed.

EXAMPLE 6

Large batch cokings were made on samples of a different fresh feed batchof decant oil feestock from the same supplier as in No. 1, with resultsas follows:

    ______________________________________                                                    %                                                                             Cr.sub.2 O.sub.3                                                              added                                                                         to            Coke Properties                                                   Feed-   Coke    CTE                                             Wt.           stock   Yield   10.sup.-7 /°C.                                                                DP, % Δ L                          ______________________________________                                        Run No's                                                                             991#                                                                   2, 3, 7                                                                              (450 kg.)                                                              composite         0       21.2% 3.2    7.8                                    Run No's                                                                      4, 5, 6, 8                                                                    composite                                                                            1303 (591 kg.)                                                                           0.11    22.2  4.3    0.7                                    Run No's                                                                      10, 11, 12                                                                    composite                                                                            1028 (466 kg.)                                                                           0.15    21.2  5.1    0.1                                    ______________________________________                                    

All of the above, except #5 were bubbled with N₂ through the 400°-440°C. temperature range at the rate of 2 C.F.M. (57 l./min.).

The composite samples were calcined at about 1300° C. for 30 min. withcalcined coke yields of about 75%. The properties of the calcined cokeswere as follows:

    ______________________________________                                                Inhibitor                                                                              Density  Ash  CTE                                            Sample No.                                                                            %        g/cc     %    10.sup.-7 /°C.                                                                DP, % Δ L                         ______________________________________                                        1894-20A                                                                              0        2.10     0.24 3.9    1.1                                     1894-20B                                                                              0.11     2.14     0.81 4.6    0.3                                     1894-20C                                                                              0.15     2.12     1.23 4.3    0.1                                     ______________________________________                                    

The values for the raw and calcined cokes were in good agreement, exceptfor the uninhibited calcined sample, which had a lower than expecteddynamic puffing, which ordinarily is similar to the puffing shown by theraw coke exam.

EXAMPLE 7

In a test to determine the appropriate amount of Cr₂ O₃ to be added tothe feedstock and the amount of agitation, an automatic coker wascharged with 10 kg of another lot of the previously mentioned decant oilfrom Example 6 and varying amounts of Cr_(O) ₃ were added to thefeedstock. The batches were agitated by bubbling with 2 C.F.H. (57l./hr.) N₂ through the temperature interval from 390°-440° C., which atthe temperature rise of 5°/C./hr, took 10 hrs. Results are tabulatedbelow:

    __________________________________________________________________________                                 Coke Properties                                  % Cr.sub.2 O.sub.3 added                                                                   Coke                                                                              Ash Distribution                                                                          CTE  DP                                          Run No.                                                                            to feedstock                                                                          Yield                                                                             Top Mid Bot.                                                                              10.sup.-7 /°C.                                                              % Δ L                                 __________________________________________________________________________    1897-21                                                                            0      31.1%                                                                              0.03%                                                                             0.01%                                                                             0.02%                                                                             ˜0.1                                                                         14.9                                        1897-22                                                                            0.1    31.8 0.34                                                                              0.28                                                                              0.33                                                                              2.8  4.4                                         1897-35                                                                            0.2    28.0 0.67                                                                              0.66                                                                              0.84                                                                              4.4  <-0.5                                       1897-20                                                                            0.4    31.6 1.19                                                                              1.26                                                                              1.28                                                                              4.1  <-0.1                                       __________________________________________________________________________

Low CTE values characterize graphite bodies made from high sulfur needlecokes because such bodies puff considerably (see 1897-21) attemperatures between 1200°-2900° C. due to the high sulfur content. Thecokes above showed no puffing when 0.2% Cr₂ O₃ was added to thefeedstock while maintaining a CTE within the level of a high qualitycoke.

EXAMPLE 8

The same feedstock in Example 7 was processed in a 4 l. resin flask with0.5 C.F.H. (14 l./hr.) N₂ bubbled through the system during thetemperature interval of 400°-450° C. while the temperature was raised ata rate of 20°/hr, for a period of 2.5 hrs. Results of these tests areshown below:

    ______________________________________                                               Cr.sub.2 O.sub.3                                                              Add. to  Coke    Ash   Ash   CTE    DP                                 Run No.                                                                              feedstock                                                                              Yield   Top   Bot.  10.sup.-7 /°C.                                                                % Δ L                        ______________________________________                                        1959-7/10                                                                            0.15%    26.1%   0.56% 0.58% 3.1      0.3                              1959-3/5                                                                             0.2      24.5    0.82  0.89  3.8    <-0.7                              ______________________________________                                    

It is evident here also that a high quality coke with low CTE andpuffing has been made.

EXAMPLE 9

In this test, coke samples produced in Examples 7 and 8 above were mixedin a standard commercial mix of coke with particles up to 3 mesh(1.2/cm.) with coal tar pitch binder, forming pieces by molding, bakedto 720° C., then graphitized at two different upheat rates of 5° and 14°C./min., with results showing dynamic puffing over the range of1200°-2900° C. as follows:

    ______________________________________                                                          % Cr.sub.2 O.sub.3                                                 Cr.sub.2 O.sub.3 Added                                                                   in coke    DP, % Δ L                                                                       DP, % Δ L                          Coke   to Feedstock                                                                             (Composite)                                                                              5° C./min.                                                                     14° C./min.                       ______________________________________                                        1897-21                                                                              0          0          9.4     21.0                                     1897-22                                                                              0.1        0.34       4.5     7.0                                      1859-7/10                                                                            0.15       0.67       0.8     1.6                                      1897-20                                                                              0.4        1.25       0.1     0.4                                      ______________________________________                                    

The improvement due to Cr₂ O₃ in elimination of puffing at the higherupheat rate of 14° C./min. is particularly evident in the above data andin the attainment of protection against puffing at a low inhibitorlevel.

EXAMPLE 10

Blends of smaller particle sized (up to 6 mesh ((2.4 mesh/cm.))) cokesmade in two different cokers from the same feedstock as in Example 7,mixed with coal tar pitch binder and molded as in Example 9, were testedat the two upheat rates, with the following results:

    __________________________________________________________________________          % Cr.sub.2 O.sub.3                                                            Added to                                                                            Coke % Ash                                                                              CTE  DP, % Δ L                                                                     DP % Δ L                               Run No.                                                                             Feedstock                                                                           % Yield                                                                            in Coke                                                                            10.sup.-7 /°C.                                                              5° C./min.                                                                   14° C./min.                           __________________________________________________________________________    1897-35                                                                             0.2   28   0.73 4.4  0.6   0.8                                          (Automatic                                                                    Coker)                                                                        1959-3/5                                                                            0.2   24.5 0.92 3.8  0.2   0.5                                          (Resin                                                                        Flask)                                                                        __________________________________________________________________________

(The higher ash content of the sample prepared in the resin flask isprobably due to a slight pickup of silica from the flask.) The favorableresults obtained at the high upheat rates are notable.

EXAMPLE 11

Chromite ore ground to -325 mesh (125 mesh/cm.) was dispersed in thesame decant oil coker feedstock and the mixture was coked, in the samefashion as in Example 1 above. The ore had the following analysis:

Cr₂ O₃ -36%

Al₂ O₃ -30%

Fe₂ O₃ -14%

inert-20%

Particle size-<325 mesh (125 mesh/cm.)

The results were as follows:

    ______________________________________                                        % Ore Added   CTE                                                             to Feedstock  10.sup.-7 /°C.                                                                      DP, % Δ L                                    ______________________________________                                        0             2.9          8.3                                                0.1           3.8          1.3                                                0.2           3.2          0.5                                                0.3           5.7          <-2.3                                              ______________________________________                                    

The data shows that chromite ore as the source of chromium is effectiveas a puffing inhibitor.

EXAMPLE 12

In this run, two feedstocks with very high sulfur levels were treatedwith Cr₂ O₃ and coked by the standard method in Example 1, and comparedwith untreated control samples. Results were:

    ______________________________________                                                     Coke Properties                                                           % Cr.sub.2 O.sub.3                                                            Added to               CTE                                           Feedstock                                                                              Feedstock Ash %   S %  10.sup.-7 /°C.                                                                DP, % Δ L                        ______________________________________                                        Clarified Oil                                                                          0         0.10    2.30 3.0    4.4                                             0.4       1.35    2.34 7.3    0.2                                    Decant Oil                                                                             0         0.26    2.84 0      12.9                                            0.4       1.80    2.84 8.3    0.5                                    ______________________________________                                    

The CTE of the cokes produced were excessive for premium needle cokes tobe used in making ultra high power graphite electrodes. The very highsulfur contents of the oils necessitated the high level of Cr₂ O₃addition, which reduced puffing to a satisfactory level, but increasedthe CTE to a point above the acceptable range for such an application.The cokes were suitable, however, for making graphite bodies andelectrodes for less severe applications.

EXAMPLE 13

Two samples of decant oils from another source were coked and tested asin Example 1 with the following results:

    ______________________________________                                        % Cr.sub.2 O.sub.3       Coke Properties                                              Added to               CTE                                            Sample No.                                                                            Feedstock Ash %   S %  10.sup.-7 /°C.                                                                DP, % Δ L                         ______________________________________                                        893     0         0.18    1.05 1.6    6.9                                     893     0.1       0.63    1.14 2.9    0                                       363     0         0.03    1.08 1.5    6.2                                     363     0.1       0.50    1.09 2.4    0                                       ______________________________________                                    

It is evident that the above cokes have desirably low CTE and dynamicpuffing characteristics.

EXAMPLE 14

The same fresh feed used in Example 7 was tested as in Example 1 usingthe inhibitor at a higher level of addition, with the following results:

    ______________________________________                                        Inhibitor      Coke Properties                                                Added to       CTE                                                            Feedstock      10.sup.-7 /°C.                                                                  DP, % Δ L                                       ______________________________________                                        0              2.9      8.3                                                   Cr.sub.2 O.sub.3 (0.2%)                                                                      4.2      0                                                     ______________________________________                                    

The utility of Cr₂ O₃ in the above test is evident.

In the above examples, it was noted that bodies produced from the cokesamenable to inhibition displayed expansion maxima at temperatures in theregion of 2500°-2600° C. Another advantage in the use of Cr₂ O₃ ascompared to other inhibitors is shown in the improvement in secondarypuffing, wherein the expansion curve for Cr₂ O₃ inhibited cokes turnsdownward at temperatures of 2700° C. and higher, while the Fe₂ O₃inhibited cokes show a continued expansion at this temperature.

Some feedstocks may well need and be beneficially treated with Cr₂ O₃additions of as much as 0.5%, resulting in a 2% ash level of Cr₂ O₃ inthe final coke.

The examples above are not shown as limitations but merely samples fromthe wide variety of petroleum residues currently available.

EXAMPLE 15

During operation of a pilot coker, it was observed that Cr₂ O₃, wheninjected into the feed line, had segregated in the bottom in severalpockets. Although it is known that space velocity in a commercial cokeris much higher than in a pilot coker, it was believed that this mightnot be sufficient to maintain dispersion of Cr₂ O₃ uniform from top tobottom. Consequently, during the next run the Cr₂ O₃ dispersion wasinjected into the top of the coker through an anti-foam injector port asa 5% dispersion in slurry oil cut back 25% with VM&P naphtha, making theCr₂ O₃ concentration 3.75% in the total additive. The resulting ashlevels were as follows, from vertical and horizontal sections:

    ______________________________________                                               Ash %                Ash %                                             ______________________________________                                        Top      0.35         Outer edge                                                                              0.29                                          Middle   0.28         Outer-mid 0.28                                          Bottom   0.31         Inter-mid 0.31                                                                Center    0.34                                                                Core      0.33                                          ______________________________________                                    

The calculated ash (Cr₂ O₃) in the coke was 0.35% on the basis of theinjected amount, with these figures showing a uniform distribution ofthe Cr₂ O₃ in the coke produced.

Tests for CTE and dynamic puffing confirmed that the coke produced wasof excellent quality:

    ______________________________________                                        CTE × 10.sup.-7 /°C.                                                                       DP                                                  No                2pph    No additive                                         Additive          Fe.sub.2 O.sub.3                                                                      %, Δ L                                        ______________________________________                                        Top     5.2           5.5                                                     Middle  5.2           5.9     +0.2%                                           Bottom  5.3           6.7                                                     ______________________________________                                    

We claim:
 1. In a process for the manufacture of non-puffing needle cokesuitable for graphitization by delayed coking of petroleum feedstockshaving a level of sulfur content combined molecularly in components ofsaid feedstock sufficiently high enough to contribute to an irreversiblevolumetric expansion of the coke on heating to a temperature of 1400° C.or higher, the improvement comprising the addition and dispersion of achromium compound into the feedstock prior to or during coking of thefeedstock in a delayed coker, said chromium compound being added in anamount effective to render the coke non-puffing.
 2. In a process for themanufacture of non-puffing needle coke suitable for graphitization andhaving a graphite CTE characteristic of not over 7×10⁻⁷ /°C. over therange of 0° to 50° C. by delayed coking of petroleum feedstocks having alevel of sulfur content combined molecularly in components of saidfeedstock sufficiently high enough to contribute to an irreversiblevolumetric expansion of the coke on heating to a temperature of 1400° C.or higher, the improvement comprising the addition and dispersion of afinely divided chromium compound into the feedstock prior to or duringcoking of the feedstock in a delayed coker, said chromium compound beingadded in an amount effective to render the coke non-puffing.
 3. Theprocess of claims 1 or 2 wherein from 0.05 to 0.5% of chromic oxide isadded to the feedstock.
 4. The process of claims 1 or 2 wherein from0.05 to 0.5% of chromic oxide is added to the feedstock in the form of apredispersed concentrate of said chromic oxide in a liquid medium,compatible with said feedstock.
 5. The process of claims 1 or 2 whereinchromic oxide is used which has a particle size such that 100% of saidchromic oxide particles are less than 5 microns and 70% are less than 2microns in diameter.
 6. The process of claims 1 or 2 wherein thechromium compound is added in the form of naturally occuring chromiteore.
 7. The process of claims 1 or 2 wherein the chromium compound isdispersed in a portion of the coker feedstock.
 8. The process of claims1 or 2 wherein the chromium compound is selected from the groupconsisting of the chromium compounds yielding chromic oxide when thecoke produced in the delayed coker is subsequently calcined at a hightemperature.
 9. The process of claims 1 or 2 wherein the chromiumcompound is selected from the group consisting of hydrated Cr (III)oxide, Cr (III) nitrate, (NO₃)₃, (NH₄)₂ Cr₂ O₇, metal chromates andbichromates.
 10. In a process for the manufacture of non-puffing needlecoke suitable for graphitization and having a graphite CTEcharacteristic of not over 5×10⁻⁷ /°C. over the range of 0° to 50° C. bydelayed coking from petroleum feedstocks having sulfur molecularlycombined in components of said feedstock in sufficient quantity tocontribute to an irreversible volumetric expansion on heating to atemperature of 1400° C. or higher, the improvement comprising selectingan effective amount of chromic oxide of particle size less than 5μ(5×10⁻⁶ m) to be added to said feedstock in a concentration by wt. of0.05 to 0.5% of said feedstock, dispersing said chromic oxide in aportion of said feedstock or other compatible material to form a uniformdispersion of said chromic oxide, adding said dispersion to theprincipal portion of said feedstock through the top of a delayed cokerand coking said feedstock.